Don't use the captured length as the packet length.

Add a routine ber_tvb_new_subset_length() used to construct tvbuffs for
parts of the packet, and have it check the specified length against the
*reported* length.  NOTE: that routine should really take an item and an
expert info value as arguments and, if the length is greater than the
remaining packet length, add an expert info for that.

Also, when counting items in a SEQUENCE-OF:

	keep track of whether we succeeded in counting them, and report
	an "unknown number of items" if we didn't;

	if the length of an item in the SEQUENCE-OF is so big we get an
	overflow, just bail out of the count loop and indicate that we
	didn't succeed in counting them - let the error be reported in
	the process of dissection.

Change-Id: I32172737baaed35fc9a0e6c19a727a6ac71ddfb2
Reviewed-on: https://code.wireshark.org/review/4103
Reviewed-by: Guy Harris <guy@alum.mit.edu>